1. Field of the Invention
The present invention relates to a spread spectrum communication device for use in a direct sequence spread spectrum, code division multiple access system, wherein a symbol is spread by a pseudo-noise reference sequence with a chip rate which is substantially greater than a symbol rate of said symbol, so as to form a spread spectrum signal, said spread spectrum signal being modulated onto a carrier and being transmitted over an air interface tending to produce multipath components of said modulated spread spectrum signal.
The present invention further relates to a receiving method for use in a direct sequence spread spectrum, code division multiple access system.
2. Description of the Related Art
In the U.S. Pat. No. 5,648,983 a so-called rake receiver is disclosed for use in a direct-sequence spread spectrum (DSSS), code-division multiple access (CDMA) system. Said rake receiver, which in principle is a matched filter type of digital symbol receiver, comprises a plurality of so-called rake fingers for correlating delayed replica of a received signal with a reference sequence such as a locally generated pseudo-noise (PN) sequence. The received signal is a data signal, which is spectrally, de-spread by the same pseudo-noise sequence as generated in a transmitter. The rake fingers are coupled to a tapped delay line of which the delay between consecutive taps is less than a duration of an element of the pseudo-noise sequence, such an element being a so-called chip. Output signals of said rake fingers are combined so as to obtain a coherently added signal from signals that have propagated through multipath channels and that have undergone different propagation delays, said coherently added signal being dumped to a symbol detector at a rate of the symbols to be detected. In principle, a total delay of said tapped delay line is in the order of the delay spread of the different propagation delays. A sampler provides samples of the received signal to said tapped delay line, sampling being performed with sub-chip resolution, i.e., at a sampling rate greater than a chip rate of the PN-sequence. A channel estimator, being coupled to the rake receiver, actually resolves multipath components spaced apart closer than a chip interval, by estimating a channel impulse response with high resolution using de-convolution techniques. Before combining the output signals of the rake fingers are multiplied by respective estimates of the signal phase for respective paths, said estimates being obtained by a phase estimator. Such a rake receiver is thus capable of resolving multipath signals that have path components spaced closer than one chip of the reference sequence, though at the cost of a complex receiver structure with a great number of rake fingers.
In the U.S. Pat. No. 5,818,866, a method for use in a CDMA rake receiver is disclosed for selecting multiple propagation delays for the reception of messages transmitted in a spread spectrum radio communication system. The rake receiver comprises a number of reception arms or fingers. Each finger provides for the reception of the signal along a propagation path identified by a particular delay, said delays being estimated by a channel estimator. Each finger comprises a correlator formed by a buffer memory, a complex multiplier, and a summation-accumulator, the buffer memory being sized so as to store a number of samples of the received signal, and a buffer length of a number of samples corresponding to an expected maximum delay spread in the system. In write mode, the buffer acts like shift register, whereas in read mode, the buffer is read out at an address corresponding to a delay estimated by the channel estimator. The channel estimator comprises a sliding correlator determining correlation between the received signal and a reference PN-sequence. In said method, a received signal is sampled at a sampling rate greater than the chip rate of the spreading sequences so that the channel estimator can provide estimations of the complex amplitude of the response of the propagation channel for relative propagation delays at sub-chip resolution. In said method, delays for said fingers are selected from a first and a second list, respectively. The first list contains delays corresponding to central samples of multipath correlation peaks, whereas the second list contains delays corresponding to samples which are neighbors of the central samples, the samples being above a given selection threshold. Further in said method, if the number of delays in said first list is greater than the number of arms of the rake receiver, delays of the rake arms are selected from the first list for which evaluated energies are largest. If there is an insufficient number of delays in the first list to set the delays in all rake arms, additional delays are selected from the second list for which the evaluated energy is above the selection threshold. As in said U.S. Pat. No. 5,648,983, so-called multipath diversity gain is obtained by coherently adding output signals from the rake arms. In the method as disclosed in said U.S. Pat. No. 5,818,866, the number of rake arms or fingers is thus constant, all arms being assigned to delays corresponding to samples of the received signal with an energy above said given selection threshold.
Rake receivers as described in said U.S. Pat. No. 5,648,983 and No. 5,818,866 are so-called baseband direct-sequence spread-spectrum receivers, usually implemented as an integrated circuit (IC). From a cost point of view it is highly desirable to keep the chip area of such an IC as small as possible. Because such receivers are usually part of a portable communications transceiver supplied by a battery, it is further desirable that the receiver has low power consumption so that battery power is not exhausted too soon.
In the TIA/EIA Interim Standard TIA/EIA/IS-95-A, May 1995, pages 6-7 to 6-11, 6-17, 6-18, 6-22 to 6-26, 7-1 to 7-6, 7-16 to 7-20, and 7-22 to 7-24, requirements for so-called IS-95 mobile radio station and base station operation are given so as to be able to transmit and receive CDMA direct sequence spread spectrum signals at a radio interface. On page 6-7, reverse CDMA channels are described for reception by a radio base station. On page 6-8, in FIG. 6.1.3.1-2., a reverse CDMA channel structure is given. On page 7-2, in FIG. 7.1.3.1-1., an overall structure of forward CDMA channels is given for reception by a mobile base station. The reverse CDMA channel is composed of access channels and reverse traffic channels, all of these channels sharing the same frequency radio channel using CDMA direct-sequence CDMA techniques, such a radio channel having a bandwidth of 1.23 MHz. Each traffic channel is identified by a distinct user long PN-code sequence. Data transmitted on a reverse CDMA channel is grouped into 20 ms frames. All data on the reverse CDMA channel, after convolution encoding and interleaving, is modulated by a 64-ary orthogonal modulation, and direct-sequence spread prior to transmission at a carrier. As can be seen in FIG. 6.1.3.1.-2., direct-sequence spreading is done using by modulo-2 addition of Walsh chips and said user long code sequence, such direct sequence spreading being followed by quadrature spreading using an in-phase and a quadrature pseudo-noise sequence, respectively, the quadrature sequences being periodic with period 215 chips. The spread chips are baseband filtered before being modulated onto a carrier. After interleaving, the code symbol rate is constant, in the IS-95-A system 28,800 sps. Six code symbols are modulated as one of 64 modulation symbols for transmission. As described on page 6-17, the modulation symbol is one of 64 mutually orthogonal waveforms generated using so-called Walsh functions. The PN chip rate is 1.2288 Mcps, each Walsh chip being spread by four PN chips. The long code is unique to a mobile station, whereas Walsh orthogonal modulation is applied to distinguish CDMA channel transmitted at a given radio frequency. In the forward CDMA channel structure, CDMA code channels such as a pilot channel, a synchronization channel, paging channels, and a number of forward traffic channels are defined, said code channels being orthogonally spread by an appropriate Walsh function, followed by direct sequence spreading using a quadrature pair of PN sequences at a fixed chip rate of 1.2288 Mcps. The spreading of the forward channel is done differently than in the reverse channel. Code channel zero is usually assigned to the pilot channel so that mobile can easily find the pilot channel, 75 repetitions of the pilot channel occurring every 2 seconds. Pilot channels are unmodulated channels, different pilot channels being distinguished by different timing offsets within a master clock in the CDMA system. Both on the forward and the reverse channel, after baseband filtering, quadrature signals are mapped into four phases of the carrier.
In the handbook, xe2x80x9cDigital Communicationsxe2x80x9d, J. G. Proakis, McGraw-Hill Book Company, 1989, pp. 862-872, time synchronization of spread spectrum systems is described, such time synchronization being split in two phases, an initial acquisition phase and a tracking phase after signal timing has been initially acquired. Time synchronization has to be so accurate that the PN-sequence is time synchronized to within a small fraction of the chip interval. On page 863, a sliding correlator is described to establish initial synchronization. On page 867, in FIG. 8.5.5, a delay-locked loop is shown for PN sequence tracking, such DLL tracking being described on pages 868 and 869.
It is an object of the invention to provide a spread spectrum communication device in which, at a sub-chip resolution, multipath components of a received modulated spread spectrum signal are efficiently combined, and in which, preferably, a signal-to-noise ratio in a received signal is optimized.
It is another object of the invention to provide a spread spectrum communication device in which power is saved if a number of resolved multipath components, at chip resolution, is less than a number of rake fingers in a rake receiver comprised in the spread spectrum communication device.
It is still another object of the invention to provide a spread spectrum communication device to selectively provide sub-chip resolved multipath components to single fingers of the rake receiver.
It is yet another object of the invention to estimate an average phase on sub-chip resolved multipath components to be supplied to a single rake finger.
In accordance with the invention, a spread spectrum communication device is provided for use in a direct sequence, code division multiple access system, wherein a symbol is spread by a pseudo-noise reference sequence with a chip rate which is substantially greater than a symbol rate of said symbol, so as to form a spread spectrum signal, said spread spectrum signal being modulated onto a carrier and being transmitted over an air interface tending to produce multipath components of said modulated spread spectrum signal, said spread spectrum communication device comprising:
a receiver front-end means for receiving said modulated spread spectrum signal;
a carrier demodulation means for demodulating said received modulated spread spectrum signal;
a sampling means for obtaining samples from said demodulated spread spectrum signal, said sampling means sampling having a sampling rate exceeding said chip rate;
a channel estimator for estimating from said samples, with a sub-chip resolution, of channel characteristics of said multipath components, and for determining of local maximums in said channel characteristics, and, within a chip period, of sample locations corresponding to said local maximums;
a rake receiver, said rake receiver being coupled to said channel estimator, receiving said samples, and comprising a plurality of receiver branches, each of said receiver branches comprising:
a down-sampler for sampling down said samples, on the basis of said determined sample locations, and
a correlation means for correlating said down-sampled samples with a locally generated pseudo-noise reference sequence so as to generate correlation values; said rake receiver further comprising:
combining means for weightedly combining said correlation values; and
decision means for deciding about a received symbol value, on the basis of said weightedly combined correlation values.
Because of combining sub-chip resolved multipath components in single arms of the rake receiver, typically, at the same signal-to-noise ratio of a combined signal, the rake receiver has fewer arms than known rake receivers. When implementing the spread spectrum communication device of the present invention in an integrated circuit, thus a reduced integrated circuit area can be obtained. Such an integrated circuit can be manufactured at reduced costs.
Preferably, power to unused rake fingers is switched off.